code: def to_mime_key(d):
      """convert dict with v3 aliases to plain mime-type keys"""
      for alias, mime in _mime_map.items():
          if alias in d:
              d[mime] = d.pop(alias)
      return d
  
file_path: nbformat/nbformat/v4/convert.py
imports: from __future__ import annotations
  import json
  import re
  from traitlets.log import get_logger
  from nbformat import v3, validator
  from nbformat.corpus.words import generate_corpus_id
  from nbformat.notebooknode import NotebookNode
  from .nbbase import nbformat, nbformat_minor
label_desc: Use image node_id 5 for calling a global function with example usage: to_mime_key(d) and returns: d